Polytetrafluoroethylene (PTFE) is widely regarded as one of the most universally chemically resistant chemical compounds. PTFE films are manufactured by special techniques well known in the art, such as molding-skinning, and are commercially available. They are not well suited, however, for use in fabricating protective clothing. Such films tear easily, and accordingly, often are damaged as they are fabricated into clothing. In handling chemicals or cleaning up spills, a person typically encounters a variety of tools and other objects with sharp or protruding edges. In other words, protective clothing must be capable of withstanding considerable wear and tear and fairly rough use, and PTFE films simply are not durable enough. Increasing the thickness of the PTFE film, while increasing the tensile strength of the film, is an inadequate solution because the natural stiffness of PTFE makes the film less flexible and introduces problems, especially splitting, during fabrication and wear of protective clothing.
Accordingly, PTFE films frequently are applied to both sides of a fabric substrate, such as a woven polyaramide or polyimide fabric. The resulting composite structure is more rugged than PTFE films alone. The fabric substrate acts to reinforce the PTFE film. Thinner PTFE films generally may be used, and thus, problems associated with stiffness of the film, although not eliminated, are reduced. Moreover, if the exterior PTFE film is punctured, the fabric substrate may be capable of preventing the puncture of the inner PTFE film, preserving the structural integrity of the barrier as a whole.
Such composite structures, however, are far from perfect. PTFE films are not self-adhering to fabric substrates. They also adhere poorly to most conventional adhesives, and in fact, Teflon.RTM. PTFE material sold by E. I. duPont de Nemours & Co., Inc., Wilmington, Del., is widely used as a coating to prevent sticking of adhesives to machine rolls and parts. The surface of the PTFE film, however, may be chemically etched to permit the use of some high performance adhesives, such as epoxy and acrylic adhesives. Unfortunately, being subject to a variety of mechanical forces, adhesives often permit separation of the PTFE film from the fabric substrate during fabrication and use of the protective clothing. This problem is exacerbated if the outer PTFE film has split or has been punctured, thereby permitting potentially deleterious chemicals to penetrate the composite structure and destroy the adhesive.
Alternately, the PTFE film may be formed on the fabric substrate by successive coatings of PTFE emulsions. An excessive amount of PTFE, however, is required to produce an effective, pin-hole free chemical barrier, and the resulting composite structures typically are heavy and stiff.
Moreover, if the outer PTFE film is split or punctured and chemicals penetrate into the fabric substrate, many chemicals tend to wick through the fabric substrate and contaminate areas removed from the point of penetration. If the garment is to be repaired, larger portions of the composite material must be replaced, thereby increasing the cost of the repair and compromising unnecessarily the structural integrity of the garment.
Conventional antiwicking agents typically are not universal in their action. For example, silicone based antiwicking agents resist wicking of aqueous solutions, but not of organic solvents. Other antiwicking agents suffer the additional defect of having to be applied in amounts that create unacceptable stiffness.
It also is important, of course, that the effectiveness of the barrier not be compromised at the seams of the garments. In many applications, sewn seams are preferred because of their strength, but in sewing, holes are made in the barrier which are not leak proof until sealed. A number of approaches to sealing seam holes are utilized by workers in the art. Rubber, self-sealing tape can be placed between the portions to be sewn together whereby the self-sealing tape provides the primary seal in the seam holes. Alternately, a rubber sealant can be applied over the stitching to fill the holes. Yet another approach is to cover one or both sides of the seam with a heat sealable the consisting of a PTFE film coated on one side with an adhesive, such as silicone, or a thermoplastic fluoropolymer, such as fluorinated ethylene/propylene copolymers (FEP). Self-sealing tape, rubber sealants, and silicone adhesives, however, are not sufficiently chemically resistant, and when sealants or heat sealable tape is used, additional processing steps must be used in fabricating protective clothing.
The subject invention, therefore, is directed to composite structures and garments produced therefrom in which a PTFE barrier is securely adhered to a fabric substrate in a manner such that the composite structure not only may be successfully fabricated into garments with a low incidence of tearing or damage, but also such that the composite structure maintains its integrity during the life of the garment. It also is directed to composite structures and garments produced therefrom which provide light, flexible, universally chemically resistant barriers. It is directed further to composite structures and garments produced therefrom, the seams of which may be more efficiently formed and sealed and in such a manner as to provide a durable seal. Finally, the subject invention is directed to composite structures wherein the wicking of chemicals accidentally coming into contact with the fabric substrate is minimized, and wherein the antiwicking agent is more universally effective and does not decrease significantly the flexibility of the composite structure.